src/plugins/sensors/CMakeLists.txt enables the dummy plugin if no plugins are set

if(NOT SENSORS_PLUGINS OR "dummy" IN_LIST SENSORS_PLUGINS)

in Qt 5 this condition was only applicable when explicitly selected

contains(SENSORS_PLUGINS, dummy):SUBDIRS += dummy

The dummy plugin appears to have adverse effects on performance on linux. It causes constant CPU usage in for example KDE's background daemon, without having any use. The condition should be fixed to only build when dummy was explicitly enabed.